Dual ion beam assisted deposition of biaxially textured template layers

ABSTRACT

The present invention is directed towards a process and apparatus for epitaxial deposition of a material, e.g., a layer of MgO, onto a substrate such as a flexible metal substrate, using dual ion beams for the ion beam assisted deposition whereby thick layers can be deposited without degradation of the desired properties by the material. The ability to deposit thicker layers without loss of properties provides a significantly broader deposition window for the process.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.W-7405-ENG-36 awarded by the U.S. Department of Energy. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to a process and apparatus for thepreparation of bi-axially textured template layers for coated conductortype superconductors.

BACKGROUND OF THE INVENTION

There is great interest in the high temperature superconductor (HTS)coated conductor community to develop economically scalable processesfor fabricating bi-axially textured templates on which high qualityYBa₂Cu₃O_(7−Δ) (YBCO) can be heteroepitaxially deposited. In order toachieve good superconducting properties, YBCO grains require goodalignment between each other to obtain high (>1 MA/cm²) critical currentdensities (J_(c)). The two competitive processes to produce the bi-axialtexture required by YBCO have been Roll-Assisted BI-axial Texturing ofSubstrates (RABITS) and Ion-Beam Assisted Deposition (IBAD).

The latter technique has been used in the development of IBAD depositedyttria-stabilized zirconia (YSZ) for coating meaningful lengths oncommercially important metal substrates. Further efforts have resultedin development of a process, coupled with pulsed laser deposition (PLD)YBCO, that has produced meter lengths of superconducting wire withcritical current densities over 1 MA/cm² and critical currents over 100A. Despite these results, one criticism of the IBAD-YSZ process has beenthat the time required to deposit the material with sufficient in-planetexture for high quality YBCO is too long. In order to develop texture,YSZ requires a thickness of between 0.5 and 1 micrometer (μm) to achievea Δφ (or full width at half maximum of the φ-scan peak) better than 12°.Reported IBAD deposition times have ranged from about one to twelvehours per meter of tape. Thus, the viability of this process has beenquestionable for cost efficient, industrial fabrication.

Subsequently, it has been shown that magnesium oxide (MgO) can bedeposited with the IBAD process and produce a thin film with in-planetexture comparable to YSZ that was only 10 nanometers (nm) thick. Thistranslates to a process about 100 times faster than IBAD YSZ. Thisprocess has been applied to further development in the preparation ofHTS coated conductors. For example, short length samples (less thanabout 4 cm long) using IBAD MgO templates have been produced with J_(c)sover 1 MA/cm² (77 K) for >1.5 μm thick YBCO films.

However, IBAD MgO still has some drawbacks that detract from itsviability as a template layer for long length processing of coatedconductors. The two most detrimental limitations are (1) the degradationof in-plane texture as IBAD MgO film thickness increases beyond acritical thickness of 10 nm; and, (2) the necessity to deposit IBAD MgOfilms on very smooth (<2 nm rms) substrates.

Dong et al., Journal of Materials Research, vol. 16, pp. 210-216 (2001),have suggested a dual ion beam approach for control of texture inaluminum films, but contain no hint of the applicability to thedeposition of MgO films for the subsequent deposition of thin films suchas YBCO.

After extensive and careful investigation, the present inventors havenow developed an IBAD deposition process using dual ion beams indeposition of intermediate layers for the subsequent deposition of thinfilms such as YBCO.

SUMMARY OF THE INVENTION

In accordance with the purposes of the present invention, as embodiedand broadly described herein, the present invention provides a processof forming composite structures having a layer of oriented MgO forsubsequent deposition of other epitaxially oriented (either hetero orhomo) layers, the process including depositing an amorphous layer of anoxide, a nitride or an oxynitride material by dual ion beam assisteddeposition upon the surface of a substrate, and, depositing a layer ofan oriented cubic oxide material having a rock-salt-like structure uponthe amorphous oxide, nitride, or oxynitride material layer, the layer oforiented cubic oxide material having a thickness of from about 20 nm toabout 50 nm and having a full width at half maximum φ-scan peak of lessthan about 14°.

Further, the present invention provides an apparatus for deposition of alayer of a target material upon a buffered polycrystalline metalsubstrate by dual ion beam assisted deposition, the target materiallayer having a thickness of from about 20 nm to about 50 nm and having afull width at half maximum φ-scan peak of less than about 14°, theapparatus including a substrate holder, a first ion gun for ion beamassisted deposition of a target material, said first ion gun having anincidence angle of 45° relative to a substrate, a second ion gun for ionbeam assisted deposition of a target material, said second ion gunhaving an incidence angle of 45° relative to a substrate and 90°relative to said first ion gun, and, a source for providing the targetmaterial. In a preferred embodiment, the target material is MgO. Theapparatus can further include a temperature controller for heating orcooling of the substrate during deposition.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic structure of a template architecture preparedin the process of the present invention.

FIG. 2 shows a schematic diagram of an apparatus of the presentinvention.

FIG. 3 shows a plot of RHEED diffraction spot intensity versus time andXRD φ scan correlation for films prepared by the process and apparatusof the present invention.

FIGS. 4(a) and 4(B) show a comparison of normalized intensity versustime curves for both an IBAD prepared film (4 a) and a DIBAD preparedfilm (4 b).

DETAILED DESCRIPTION

An improvement in IBAD technique for development of bi-axial texture inmagnesium oxide (MgO) for use in heteroepitaxial deposition of HTS YBCOhas now been demonstrated. For distinction between processes, theacronym for the two-gun process in the following description is DIBAD(dual ion-beam assisted deposition). The prior process, i.e., the IBADprocess used a single ion beam that irradiates a sample surface with lowenergy (<1000 eV) inert gas ions concurrently with the vapor depositionof a source material. In contrast, the present invention employs two ionbeams in conjunction with the vapor source deposition.

The result of using two ion guns is to eliminate the effect of texturedegradation beyond the critical thickness of about 10 nm observed insingle IBAD MgO. It has been observed that IBAD MgO shows a distinctbehavior in the development of in-plane texture. A detailed study by thepresent inventors has confirmed the observation of Wang et al.,“Ion-beam-induced Texturing in Oxide Thin Films and its Applications”,in Materials Science and Engineering, Stanford: Stanford University,1999, pp. 113, that texture degradation begins to occur after about 10nm of IBAD MgO is deposited.

While not wishing to be bound by the present explanation, it is believedthat the mechanism of texture degradation is as follows. As thethickness of a deposited film increases beyond the critical amount(about 10 nm), the accumulated dislocation density increases along theion beam incident direction and begins to tilt the crystal planes awayfrom the ion beam. Further deposition exacerbates this effect andresults in degradation of bi-axial texture for the film. By use ofDIBAD, it is believed that reducing the degrees of freedom available todislocation generation and movement along these specific planes canmitigate the effect.

Monitoring the change in bi-axial orientation of the IBAD MgO filmduring growth may be accomplished using Reflected High-Energy ElectronDiffraction (RHEED). In such a process, the spot intensity varies as afunction of IBAD MgO film thickness. The maximum spot intensity has beenobserved at a thickness of about 10 nm. Additional analysis hasdetermined a correlation between in-plane texture and the spot intensityby comparing different film thicknesses at points in time before, at,and beyond the observed maximum spot intensity. The best in-planetexture was obtained at the maximum spot intensity and degraded rapidlybeyond that point. This degradation is shown in FIG. 3. A similar studywas conducted for DIBAD MgO samples and no degradation was observed forfilms with thicknesses greater than 10 nm. The effect of DIBAD can bemost readily observed by examining the difference between spot intensityversus time curves as shown in FIG. 4. DIBAD MgO films have beendeposited with Δφ=9° without process optimization. This is a significantimprovement over conventional IBAD MgO processing where the processingwindow for good in plane texture (Δφ=8 to 10°) may be only severalseconds in width and difficult to predict.

Another main concern for conventional IBAD processing of MgO has beenthe need for ultra-smooth (<2 nm root mean square (RMS)) surfaces toimprove in-plane texture. It had been previously demonstrated thatdecreased surface roughness decreased in-plane misorientation andincreased subsequent YBCO J_(c) values (Groves et al., “Development ofthe IBAD MgO Process for HTS Coated Conductors”, Proc. Int. Workshop onSuperconductivity, Honolulu, Hi., p. 43 (2001)). While just increasingthe thickness of the IBAD MgO layer would seem to overcome thislimitation in the IBAD process, conventional IBAD MgO texture degradesas the thickness is increased beyond about 10 nm. It has now been foundthat this problem can be overcome by using the DIBAD process. Presently,metal tapes that have been mechanically polished with a 1 micron orfiner diamond paste for a short time period of from about 10 seconds toabout 20 seconds to get the surface roughness to about 4 nm to about 6nm RMS have been used as substrates for DIBAD MgO deposition with goodresults.

Initial results of subsequently deposited YBCO on these DIBAD MgO basedtemplates are good. The substrate used was a Hastelloy C276 hightemperature Ni-alloy substrate. Silicon nitride was deposited as anamorphous layer as taught by Do et al. in U.S. Pat. No. 6,190,752. Alayer of MgO was then deposited by DIBAD upon the silicon nitride to athickness of about 40 nm without any degradation of the MgO layer. Thesuperconducting transition temperature (T_(c)) was measured as 88.1 K.X-ray φ-scans of the YBCO had a Δφ of 13.7° with a low background count.Four microbridges were patterned on the substrate and the measurementsfor the critical current density (J_(c)) as well as the YBCO thicknessare summarized in Table 1. A typical architecture is shown in FIG. 1where the article 10 includes a polycrystalline metal substrate 12, anamourphous nucleation layer 14, an IBAD MgO layer 16, a pulsed laserdeposition buffer layer 18, and a superconducting layer 20. Ahomo-epitaxial layer of MgO can be deposited onto the IBAD MgO layer 16before buffer layer 18. TABLE 1 Bridge YBCO thickness (μm) J_(c)(MA/cm²)A 0.90 0.22 B 0.70 0.20 C 0.64 0.26 D 0.73 0.31

The present invention presents an improved approach to the deposition ofbi-axially textured MgO thin films using ion-beam-assisted deposition.This process (DIBAD) uses two ion guns and a vapor source to produce abi-axially oriented MgO template layer for the deposition of subsequentlayers. The DIBAD process eliminates several problems with the standardsingle gun IBAD deposition of MgO. First, degradation of in-planetexture after a critical thickness does not occur with DIBAD as it doeswith IBAD. Secondly, the thickness of the film can be increasedsubstantially (up to at least 5 times) beyond the critical thicknessobserved for IBAD MgO films thereby providing an industrially importantlonger processing window. Also, initially DIBAD deposited MgO filmssubsequently overcoated with YBCO have demonstrated high Tc values andfairly good J_(c) values.

In the present invention, the initial or base substrate can be, e.g.,any polycrystalline material such as a metal or a ceramic such aspolycrystalline aluminum oxide or polycrystalline yttria-stabilizedzirconia (YSZ). Preferably, the substrate can be a polycrystalline metalsuch as nickel. Alloys including nickel such as various Hastelloymetals, Haynes metals and Inconel metals are also useful as thesubstrate. The metal substrate on which the superconducting material iseventually deposited should preferably allow for the resultant articleto be flexible whereby superconducting articles (e.g., coils, motors ormagnets) can be shaped. As such a metal substrate can have a roughsurface, it had previously required much mechanical polishing,electrochemical polishing or chemical mechanical polishing to provide asmoother surface (less than about 2 nm RMS) prior to IBAD deposition.With DIBAD such a high degree of polishing is generally not needed.Substrates with 4 nm RMS have been successfully used.

Whether the metal substrate is polished or not, a layer of an inertmaterial can be deposited upon the base substrate. By “inert” is meantthat this material does not react with the base substrate or with anysubsequently deposited materials. Examples of suitable inert materialsinclude aluminum oxide (Al₂O₃), yttrium oxide (Y₂O₃), silicon nitride(Si₃N₄), and aluminum oxynitride (AlON). The inert layer can bedeposited on the base substrate by pulsed laser deposition, e-beamevaporation, sputtering or by any other suitable means. The layer isdeposited at temperatures of generally greater than about 400° C.

The ion source gas in the DIBAD process, i.e., the dual ion beamassisted deposition can be any inert gas but is preferably argon. Thedual ion beam assisted deposition is conducted with substratetemperatures of generally from about 20° C. to about 100° C. A MgO layerdeposited by the DIBAD process can generally be from about 20 nm toabout 80 nm in thickness, preferably about 20 nm to about 50 nm.

After deposition of the MgO (or other oriented cubic oxide materialshaving a rock-salt-like structure), an additional thin homo-epitaxiallayer of the same material can be optionally deposited by a process suchas electron beam or magnetron sputter deposition. This thin layer cangenerally be about 25 nm in thickness. Deposition of the homo-epitaxiallayer by such a process can be more readily accomplished than depositingthe entire thickness by dual ion beam assisted deposition.

The present invention is more particularly described in the followingexample which is intended as illustrative only, since numerousmodifications and variations will be apparent to those skilled in theart.

EXAMPLE 1

Magnesium oxide has a cubic rock-salt structure with a lattice constantof a=0.421 nm. In order to achieve bi-axial texture an amorphous layercan be deposited on a substrate surface.

The substrates used here were nickel-based alloys. Before deposition,the metal substrates were mechanically polished to an average surfaceroughness of 4 mm. An amorphous layer (about 5 nm) was deposited uponthe substrate using electron beam deposition. A subsequent layer of MgOwas deposited upon the amorphous layer using DIBAD. Argon ions wereaccelerated to 750 eV with a total current density of 100 μA/cm² usingtwo Kaufman ion sources (each ion gun provides an individual currentdensity of 50 μA/cm²). The incidence angle of the ion sources was 45°relative to the substrate that corresponds to the MgO <110>.Concurrently, an electron beam evaporator provided the magnesium oxidevapor flux at 0.15 nm/s during DIBAD growth. The ion to atom ratio wasmaintained constant at 0.7. The vapor flux and the ion fluence weremonitored with a quartz crystal microbalance (QCM) and a Faraday cup,respectively. All IBAD depositions were performed at room temperature.

IBAD film growth was monitored in situ using RHEED by collecting a spotintensity versus time (I vs. t) curve that used the reflectionscorresponding to the (002) and (022) planes. Images were captured usingkSA400 software (k-Space Associates, Ann Arbor, Mich.). All patternswere taken at the beam energy of 30 keV. A schematic diagram used isshown in FIG. 2 of the apparatus 30 which includes source 32, first iongun 34 and second ion gun 36 oriented at 90° from one another, asubstrate 38, and RHEED gun 40 with phosphor screen 42 for displaying adiffraction image of the growing film on substrate 38.

Pulsed laser deposition (PLD) was then used to heteroepitaxially depositsubsequent buffer and YBCO layers. These depositions took place atsubstrate temperatures between 730° C. and 770° C. Two buffer layerswere used in this sample. The first layer was 50 nm of YSZ followed by20 nm of yttria. Both of these layers were deposited at a rate of 0.05nm/s. These buffer layers were used to obtain improved lattice matchingwith the final YBCO films. The YBCO films were deposited at a rate of 2nm/s. Metal samples were then patterned into micro-bridges with nominaldimensions of 250 μm wide by 5 mm long. Superconducting transitiontemperatures and transition widths were measured using an inductiveprobe. Transport critical current and critical current density weremeasured in liquid nitrogen temperature (75 K) and self-field using a 1μV/cm criterion.

Although the present invention has been described with reference tospecific details, it is not intended that such details should beregarded as limitations upon the scope of the invention, except as andto the extent that they are included in the accompanying claims.

1-7. (canceled)
 8. An apparatus for deposition of a layer of a targetmaterial upon a buffered polycrystalline metal substrate by dual ionbeam assisted deposition, said apparatus comprising: a substrate holder;a first ion gun for ion beam assisted deposition of a target material,said first ion gun having an incidence angle of 45° relative to abuffered polycrystalline metal substrate; a second ion gun for ion beamassisted deposition of a target material, said second ion gun having anincidence angle of 45° relative to the buffered polycrystalline metalsubstrate and 90° relative to said first ion gun; and, a source forproviding said target material.
 9. The apparatus of claim 8 wherein saidtarget material is MgO.
 10. The apparatus of claim 8 further including atemperature controller for heating or cooling a substrate duringdeposition.
 11. The apparatus of claim 8 further including a reflectedhigh-energy electron diffraction monitor.